The killer whale and the mushroom boss fights were pretty fun but this game really needs more hit feedback and better game feel in general. The small mushrooms were also pretty hard to hit and I don't know what the C key action does.
There were also a lot of weird instant kill cheap shots like with the killer whale's offscreen attack. I was also not a fan of how small the arenas were because you barely had any space to move in them. That removes depth from the game because you can only fight in close-range so there is less risk/reward involved as with larger arenas you can play in a safe way by focusing on avoiding boss' attacks and only attacking after the boss does punishable attacks.
I think allowing the player to attack in all-8 directions, adding a weak projectile attack and making the game faster would also make it more fun